RM0090: Reference manual - STMicroelectronics

RM0090 Ethernet (ETH): media access control (MAC) with DMA controller
Figure 329. Address field format
MSB 46-bit address U/L I/G LSB
Bit transmission order (right to left)
I/G = 0 Individual address
I/G = 1 Group address
U/L = 0 Globally administered address
U/L = 1 Locally administered address
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● QTag Prefix: 4-byte field inserted between the Source address field and the MAC Client
Length/Type field. This field is an extension of the basic frame (untagged) to obtain the
tagged MAC frame. The untagged MAC frames do not include this field. The extensions
for tagging are as follows:
– 2-byte constant Length/Type field value consistent with the Type interpretation
(greater than 0x0600) equal to the value of the 802.1Q Tag Protocol Type (0x8100
hexadecimal). This constant field is used to distinguish tagged and untagged MAC
frames.
– 2-byte field containing the Tag control information field subdivided as follows: a 3bit
user priority, a canonical format indicator (CFI) bit and a 12-bit VLAN Identifier.
The length of the tagged MAC frame is extended by 4 bytes by the QTag Prefix.
● MAC client length/type: 2-byte field with different meaning (mutually exclusive),
depending on its value:
– If the value is less than or equal to maxValidFrame (0d1500) then this field
indicates the number of MAC client data bytes contained in the subsequent data
field of the 802.3 frame (length interpretation).
– If the value is greater than or equal to MinTypeValue (0d1536 decimal, 0x0600)
then this field indicates the nature of the MAC client protocol (Type interpretation)
related to the Ethernet frame.
Regardless of the interpretation of the length/type field, if the length of the data field is
less than the minimum required for proper operation of the protocol, a PAD field is
added after the data field but prior to the FCS (frame check sequence) field. The
length/type field is transmitted and received with the higher-order byte first.
For length/type field values in the range between maxValidLength and minTypeValue
(boundaries excluded), the behavior of the MAC sublayer is not specified: they may or
may not be passed by the MAC sublayer.
● Data and PAD fields: n-byte data field. Full data transparency is provided, it means that
any arbitrary sequence of byte values may appear in the data field. The size of the PAD,
if any, is determined by the size of the data field. Max and min length of the data and
PAD field are:
– Maximum length = 1500 bytes
– Minimum length for untagged MAC frames = 46 bytes
– Minimum length for tagged MAC frames = 42 bytes
When the data field length is less than the minimum required, the PAD field is added to
match the minimum length (42 bytes for tagged frames, 46 bytes for untagged frames).
● Frame check sequence: 4-byte field that contains the cyclic redundancy check (CRC)
value. The CRC computation is based on the following fields: source address,
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